You would be best served to call a prop shop or professional who does this everyday for a living. Yanmar, Westerbeke and others often give some "standard" prop sizes in their manuals. Many boats however do not fit these parameters.CPowell wrote:WOW!!!! My head is spinning over the responses I have received! Thanks to all! BUT could someone sum this dissertation up and give me the best prop recommendation, size and pitch?
I have found all your replies very informative but how do I make a decision? Why did the engine supplier (Old Port Marine in Newport, RI) recommend a 15X14 three blade?
Chris
This is not as exact a science as some would have you believe and most engine manufacturers, these days, want a prop that will let you turn max rated RPM at wide open throttle, zero current and clean bottom. Even a degree of pitch the wrong way can throw you off by 300+/- RPM.
As it is, it took me two tries even with a pro, to get my prop right. Also, different props perform very differently. For example my Michigan Wheel 3 blade was a 16" X 12P and a full "Mickey Mouse Ears" style prop, as in big blades. I switched to a Campbell Sailor, same diameter, 16" with considerably LESS blade surface area, yet the pitch went from a 12P to a 9P.
The first try put me off by just about 300 RPM. Westerbeke is fairly adamant that they want a prop that will let you turn max rated RPM. I spoke at length with their inside engineers about this, trying to avoid a haul out and prop re-pitch. In the end Campbell sent me a new prop and I hauled and installed the right combination. Engines are more costly than a short haul and prop change while still in the slings. The engine is now within 30-50 RPM of max rated.
In short not all 16" X 12P props will perform the same so you are best to involve a professional, but even then, they are often off by some margin.
These props BOTH hit max rated RPM on my boat +/- 50 RPM. The Michigan Wheel has tons more surface area and a more aggressive, in theory, 12P. The CS is the same diameter, less surface area, and a 9P.
Please ignore the incorrectly installed prop nuts. My brother was helping, and I was teaching him how to lap fit & install a prop, which he did very well with, but, I apparently forgot to mention that the small nut goes on first. I did flip flop it before launch..
16" X 9P = 2970 RPM
16 X 12P = 2985 RPM
Not all props have the same efficiency and efficiency is not always derived on blade surface area. This is why I always have my props professionally sized despite owning Dave Gerr's book and having read it front to back..
